1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an engine, which uses a reducing agent to remove, by reduction, nitrogen oxide (NOx) discharged from a diesel engine, a gasoline engine, or the like, mounted on a mobile vehicle. In particular, it relates to an engine exhaust gas purifying apparatus that prevents clogging of an injection nozzle for supplying a reducing agent to an exhaust gas on an upstream side of a reducing catalyst, and when clogging in the injection nozzle occurs, clears such clogging, to improve the efficiency of NOx purification processing.
2. Description of the Related Art
As a system which purifies exhaust gas by removing particularly NOx from among particulate matters (PM) in exhaust gas discharged from an engine, several exhaust gas purifying apparatuses have been proposed. In these exhaust gas purifying apparatuses, a reduction catalyst is placed in the exhaust system of the engine, and a reducing agent is injection-supplied into an exhaust gas passage on the upstream side of the reduction catalyst, to thereby catalytically reduction react the NOx in the exhaust gas with the reducing agent, and thus purification process the NOx into harmless constituents. The reducing agent is stored in a liquid state at room temperature in a reservoir tank, and a necessary amount thereof is injection-supplied from an injection nozzle. The reduction reaction uses ammonia which has excellent reactivity with NOx, and a reducing agent water solution such as a urea aqueous solution, ammonia aqueous solution or the like that is hydrolyzed to produce ammonia easily is used as the reducing agent (refer for example to Japanese Unexamined Patent Publication No. 2000-27627).
However, in the aforementioned conventional exhaust gas purifying apparatus, an amount of reducing agent supplied is controlled in accordance with the operation state of the engine (exhaust gas temperature, NOx discharge quantity, and so forth). However in some cases, depending on the operation state of the engine, the injection holes of the injection nozzle provided in the exhaust gas passage, or the passage leading thereto, may become clogged and it may become impossible to supply reducing agent sufficiently. As a result, the reduction reaction of the NOx with the above-mentioned reduction catalyst does not proceed smoothly, and there is a possibility of NOx being exhausted.
The above clogging of the injection nozzle is principally caused by the urea (referred to below as “solid urea”) in the urea aqueous solution (referred to below as “urea water”) serving as the reducing agent, crystallizing and solidifying in the injection holes, or the passage leading thereto. This is because urea water solidifies at 100° C., and hence when the urea water is heated above 100° C. urea crystals form. Here, during the normal injection supply of urea water by the injection nozzle, the urea water supplied from the reservoir tank, (urea water and compressed air in the case of a reducing agent supply system that supplies compressed air with urea water to the injection nozzle), cools the interior of the nozzle, so that even if the injection nozzle is heated by the exhaust gas from the engine, the urea water does not reach 100° C. However, in the case where the quantity of urea water supplied is reduced, so that the interior of the nozzle can no longer be cooled, there is a possibility that the urea water inside the nozzle could reach or exceed 100° C., and urea crystals could form, causing clogging.
Moreover the melting point of solid urea is 132° C. Therefore if the exhaust gas temperature close to the injection nozzle is raised by the exhaust gas from the engine, and the heat input to the injection nozzle is increased, the solid urea will melt and the clogging of the nozzle will be cleared.
However, in the case where a reduction agent supply means is a so-called air assisted type reduction agent supply means that supplies compressed air together with urea water to the injection nozzle to atomize the urea water and eject it, the compressed air that is constantly supplied to the injection nozzle cools the interior of the nozzle. Therefore, the temperature of the interior of the nozzle does not rise above 132° C., and melting of the solid urea is prevented. Consequently there is a possibility that solid urea that has crystallized and solidified will attached to the interior of the injection nozzle, and cause clogging of the nozzle. In this case, in order to raise the temperature of the interior of the injection nozzle to melt the sold urea, it is considered that the exhaust gas temperature inside the exhaust gas passage rises. However this may not be advisable for some engines.